The death toll from the deadliest mass shooting in US history will rise, according to US doctors, as the identities of the 50 people who were gunned down began to emerge.

So far 15 victims aged between 20 and 50 have been publicly identified as victims of Omar Mateen, 29, who massacred revellers and at least one employee at a gay nightclub in Orlando, Florida.

According to reports. the bodybuilder, who was of Afghan origin, telephoned police and pledged allegiance to Islamic State before he opened fire on the crowds with an assault rifle and handgun.

The terror group has since claimed responsibility for the attack, which was described by President Barack Obama as an "act of hate".

Meanwhile, it emerged that the New York-born killer, who worked for security giant G4S, had come under the gaze of counter-terror investigators several times in recent years.

Law enforcement officials work at the Pulse nightclub following a fatal shooting Sunday, June 12, 2016, in Orlando

More than 300 people were inside the Pulse nightclub when the sound of gunfire began to rip through the air just after 2am.

The killer moved into the nightclub, firing repeatedly, before taking dozens of revellers hostage.

Mateen was shot dead in a gunfight with Swat officers who moved in at around 5am. He had mortally wounded at least 50 victims and injured dozens more.

Officials said 39 people died in the venue and 11 at hospitals in the city where 53 people remain, most in a critical condition.

Orlando Mayor Buddy Dyer described the scene, saying there was "blood everywhere".

The majority of the casualties were taken to the Orlando Regional Medical Centre, where trauma surgeon Dr Mike Cheatham warned: "I think we will see the death toll rise."

The youngest of the dead to be named so far is Luis Omar Ocasio-Capo, 20. The oldest to be killed was Franky Jimmy Dejesus Velazquez, 50.

Kimberly Morris, 37, had moved to Orlando around two months ago from Hawaii and worked as a bouncer at the nightclub, the Orlando Sentinel reported.

Friend Lo Anthony wrote on Facebook that she "always had a smile on her face" and was "loved and adored by all".

President Barack Obama speaks about the massacre at a nightclub in Orlando that claimed 50 lives during a news conference at the White House

Mr Obama earlier called the massacre at the LGBT club as an "act of terror" and an "act of hate", describing Mateen as being "filled with hatred".

However it also served as a reminder of how easy it is for someone to access a weapon like a gun.

"We have to decide if that is the type of country we want to be. To actively do nothing is a decision as well," the president said.

Mateen was known to the FBI after making inflammatory comments to co-workers about extremism in 2013. He was questioned twice and investigated again in 2014 over potential links to an American suicide bomber, but the probe was dropped after he was deemed to be low-risk.

However he was not under surveillance and the FBI said he had legally purchased two firearms within the last week.

The 911 calls involving the gunman from Port St Lucie, Florida, which featured conversations about Islamic State before the massacre have been handed to investigators.

Mateen is believed to have pledged allegiance to IS leader Abu Bakr al-Baghdadi during the call.

IS's Amaq news agency is understood to have issued a statement saying the massacre was "carried out by an IS fighter".

This undated image shows Omar Mateen, who authorities say killed dozens of people inside the Pulse nightclub in Orlando

However US officials said it was possible that he was self-radicalised but not "under the command and control of Isis".

An ex-wife described Mateen, reportedly a regular at an Islamic centre in the city, as a violent bodybuilder who had a history of abusing steroids.

Meanwhile his father, Seddique Mir Mateen, told NBC news that his son had recently been angered after seeing two men kissing while out with his young son and denied the killings had a religious motive.

On Sunday evening vigils were held in Orlando and a group of around 100 people gathered in Paris in a show of solidarity.

British comedian James Corden, who hosted the Tony Awards in New York on Sunday evening, paid tribute to the victims and their loved ones, saying: "Your tragedy is our tragedy."

Prime Minister David Cameron earlier said he was "horrified" by the shootings, while Afghanistan's President Ashraf Ghani said "targeting civilians is not justifiable under any circumstances whatsoever".

French President Francois Hollande said he "expresses the full support of France and the French with America's authorities and its people in this difficult time".

Rachel Henry, from left, Selene Arciga, Nicolette Gullickson, Joanna Lamstein join members and supporters of the LGBT as they gather for a candlelight vigil in front of the White House

And Buckingham Palace said a personal message had been sent to President Obama, saying the Queen and Duke of Edinburgh were "shocked by the events in Orlando.

"Our thoughts and prayers are with all those who have been affected."

Mr Obama ordered flags at the White House and federal buildings to be flown at half-mast as a mark of respect for the victims. The Empire State Building in New York was left in darkness as a mark of sympathy on Sunday evening.

In the wake of the slaughter, a heavily-armed Indiana man was arrested on his way to a southern California gay pride parade.

Police stopped James Wesley Howell, 20, and discovered an arsenal of weapons in his vehicle, including three assault rifles, high-capacity magazines and ammunition, and a five-gallon bucket with chemicals that could be used to make an explosive device.

The names of some of the victims of the Pulse nightclub shooting have been released by the City of Orlando, after their next of kin were contacted.

The list will be periodically updated as those who died are identified and families and loved ones notified.

The city gave no additional information. At least 50 people were killed in the attack.